We are studying the mechanisms of action, the regulation and the roles in intermediary metabolism of the enzymes involved in the conversion of thymidine to the pyrimidines of RNA in Neurospora crassa and to uracil-5-carboxylic acid in mammals. The enzyme in Neurospora which are being purified and subjected to mechanistic studies include those that catalyze the following reactions: deoxyribonucleosides yields ribonucleosides, thymine yields 5-hydroxymethyluracil, 5-hydroxymethyluracil yields 5-formyluracil, 5-formyluracil yields uracil-5-carboxylic acid and uracil-5-carboxylic acid yields uracil plus CO2. Analogous reactions are being studied with mammalian systems.